Polycarbonate resins are characterized by their transparency, mechanical strength and dimensional stability. These properties make polycarbonate an ideal choice for the preparation of articles, especially laminates or sheets, useful in application in hostile environments. One of the drawbacks of the resin used in this application is its tendency to discolor upon prolonged exposure to the heat and/or UV. Among the relevant applications, mention may be made of lighting lenses and associated parts in conjunction with metal halide, mercury vapor, high pressure sodium and other High Intensity Discharge (HID) lamps which generate UV radiation and significant heat. The tendency of polycarbonate to discolor, limits its applicability and methods to overcome the deficiency have long been sought.
It is therefore an object of the present invention to provide polycarbonate compositions which are suitable for the preparation of articles, most especially sheets and laminates, having improved resistance to discoloration caused by exposure to heat and/or UV.
The art has long recognized the efficacy of hindered phenols as antioxidants in the context of polymeric resins. U.S. Pat. No. 4,563,516 disclosed sterically hindered phenols as stabilizers of a carbonate polymer, and a hindered phenol phosphite was disclosed in U.S. Pat. No. 4,276,233 as a thermal stabilizer of polycarbonate resins. U.S. Pat. No. 4,812,498 disclosed a polycarbonate resin composition having improved resistance to deterioration when exposed to light and containing the bisbenzotriazole stabilizer of the present invention. Importantly, the possible use of the bis-benzotriazole in combination with phenolic antioxidants is disclosed in the '498 document.
The art is noted to include Canadian Patent 1,208,873 which disclosed a polycarbonate-based panel made resistant to UV radiation. Accordingly, a panel is structured to include a core layer of polycarbonate to which there is adhesively bonded an intermediate UV absorption layer and a cover layer. The purpose of the cover layer is to prevent vaporization of the UV absorber from the intermediate layer. The intermediate, UV-absorption layer may be prepared from polycarbonate and contains derivatives of benzotriazole as UV absorbers. Also noted is German Patent Application 1,670,951 which disclosed polycarbonate molded articles, including ribbons which are rendered resistant to UV radiation by incorporating the bis-benzotriazole compound of the present invention therewith. A method for coating a polycarbonate sheet with a protective layer was disclosed in UK Patent Application 2,028,228. A layer preferably of polymethacrylate and advantageously containing a UV absorber is said to be applied to the sheet by co-extrusion. U.S. Pat. No. 3,892,889 discloses UV stabilized polycarbonate moldings, the surfaces of which have been treated with a solution containing a benzotriazole. German DE-OS 3,617,978 discloses co-extruded sheets based on a polycarbonate resin which sheets are covered by a UV absorbing layer made from a branched polycarbonate resin containing the bis-benzotriazole of the present invention. Also relevant is European Patent Application 110,221 which disclosed a panel consisting of a core layer of polycarbonate containing not more than 0.5 wt. percent of a UV absorber and having on at least one side a coating layer which has been coextruded with the core and which contains at least 3 percent of a UV absorber.
U.S. Pat. No. 4,948,666 is noted to disclose a polycarbonate composition containing the bis-benzotriazole of the present invention useful in the preparation of a stain-protective layer for polycarbonate sheets. Also relevant in the present context is U.S. patent application Ser. No. 07/732,262 filed Jul. 18, 1991 which disclosed relevant technology.
The present invention resides in the finding of a particularly efficient combination of stabilizers.